Rock anchor bolts which include a hollow rod having a hole at or near their ends which are located in a hole and through which grout can be pumped to fill the holes are known. A problem with this type of bolt is, however, that there is litle or no indication of whether the bolt is fully grouted in its hole or not and it certainly happens that bolts which are thought to be full column grouted are dangerously not. A further problem with conventional rock anchors which employ mechanical expansion heads is that it is frequently difficult to engage the unexpanded anchor shells of the heads at a particular position in the holes by merely rotating the bolts to expand the heads so that difficulties are frequently experienced in locating the bolts precisely in the holes which are to house them. This is particularly so in hard rock in which the wall of the hole is smooth.
A vast number of anchor bolts are employed daily in mining and tunneling operations and their expense adds signifcantly to the mining and tunneling costs. Most known anchor bolts are tensioned in use by means of a nut which is pulled up against whatever is anchoring the rod of the bolt in the hole on a threaded end of the bolt against a washer at the mouth of the hole. Bolt threading and the provision of a nut together contribute signifcantly to the cost of the bolts. Additionally, the effective cross sectional area of an anchor bolt rod and so its tensile strength is reduced by thread cutting. This reduction in the effective cross sectional area of a bolt results in expensive waste material. To overcome this problem the ends of some bolts are upset or rolled to a larger diameter than the remainder of the bolt with the threads then being rolled into the upset portion of the bolt. Although this procedure results in less waste bolt material the cost saving is largely negated by the cost of upsetting or rolling the bolts. Another bolting problem caused by thread tensioning arrangements is that of ensuring that a bolt is correctly tensioned. As is well known, approximately 90% of the torque applied to a tensioning nut is employed in overcoming friction, 50 % at the bearing face of the nut on the roof washer and 40% between the mating bolt and nut threads. This results in only 10% of the applied effort being employed in axially loading the bolt and not even in this figure is certain.
Yet another problem with most conventional rock anchor bolts which employ expansion heads for point anchoring is that when locked into the hole, any attempt to withdraw the rod from the hole tends more firmly to wedge the anchor head in the hole. The result of this is that the bolt snaps when stretched beyond its elastic limit. This is a particularly serious problem in deep level gold mines where closure of the hanging and footwalls is inevitable and in seismic generated rockbursts.